This invention relates in general to applying heat to wells and in particular to a heater cable that is deployable while the well is live.
Occasions arise wherein it is desirable to add heat to a hydrocarbon producing well. For example, U.S. Pat. No. 5,782,301 discloses a heater cable particularly for use in permafrost regions. The heater cable in that instance is used to retard the cooling of the hydrocarbon production fluid as it moves up the production tubing, which otherwise might cause hydrates to crystalize out of solution and attach themselves to the inside of the tubing. Also, if water is present in the production stream and production is stopped for any reason, such as a power failure, it can freeze in place and block off the production tubing.
Another application involves gas wells, which often produce liquids along with the gas. The liquid may be a hydrocarbon or water that condenses as the gas flows up the well. The liquid may be in the form of a vapor in the earth formation and in lower portions of the well due to sufficiently high pressure and temperature. The pressure and the temperature normally drop as the gas flows up the well. When the vapor reaches its dew point, condensation occurs, resulting in liquid droplets. Liquid droplets in the gas stream cause a pressure drop due to frictional effects. The pressure drop results in a lower flow rate at the wellhead. The decrease in flow rate due to the condensation can cause a significant drop in production if the quantity and size of the droplets are large enough. A lower production rate causes a decrease in income from the well. In severe cases, a low production rate may cause the operator to abandon the well.
Applying heater cable to a well in the prior art requires pulling the production tubing out of the well, strapping a heater cable to the tubing and lowering the tubing back into the well. One difficulty with this technique in a gas well is that the well would have to be killed in order to pull the tubing. This is performed by circulating a liquid through the tubing and tubing annulus that has a weight sufficient to create a hydrostatic pressure greater than the formation pressure. However, in low pressure gas wells, killing the well is risky in that the well may not readily start producing after the killing liquid is removed. The killing liquid may flow in the formation, blocking return of gas flow.
The heater cable of the type in U.S. Pat. No. 5,782,301 does not have the ability to support its own weight. It must be supported by another structure, such as the production tubing. Proposals have been made for installing a coiled tubing with a heater cable located therein. Coiled tubing is a metal continuous tubing that is deployed from a reel to the well. The diameter is typically from about 2 to 2xe2x85x9e inch. Coiled tubing is normally made of a mild steel in a seam welding process. After welding, it is annealed to provide resistance to cracking as it is wound on and off a reel. produced by rolling a flat plate. If heater cable is to be located within a string of coiled tubing, it will be pulled through the cable after the annealing process because the temperatures employed during annealing would damage the insulation of the heater cable. A variety of techniques, including standoffs, dimples and the like have been proposed to cause the power cable to grip the coiled tubing to transfer its weight to the coiled tubing. Because of the standoffs, the outer diameter of the coiled tubing is larger than desirable. When deployed within production tubing, coiled tubing reduces the flow area of the production tubing, increasing pressure drop and frictional losses.
The heater cable for this invention has at least one insulated conductor. An elastomeric jacket is extruded over the insulated conductor, the jacket having a cylindrical exterior that has a longitudinally extending recess formed thereon. A metal tubing having a cylindrical inner wall and a longitudinally extending weld seam is formed around the jacket. The seam of the metal tubing is welded in a continuous process and is located adjacent the recess so as to avoid excessive heat to the jacket while the seam is being welded. The coiled tubing initially has a greater inner diameter than the outer diameter of the jacket. After welding the seam, the coiled tubing is swaged to a lesser diameter, causing its inner wall to frictionally grip the jacket.
The coiled tubing is preferably formed of a stainless steel that provides sufficient strength and toughness to be used as coiled tubing without an annealing process. Preferably, the outer diameter of the coiled tubing after swaging is no greater than one inch.